The present invention relates to a method and apparatus of preventing a flexible jacket, which is included in a roll nip formed between a shoe press roll having a concave, plane, or slightly convex press shoe, and the flexible jacket and a hot counter roll, from getting damaged by the heat from the counter roll upon a web break. The method includes the step of opening the roll nip by removing one of the press shoe and the hot counter roll from the other, whereby the jacket resumes a generally cylindrical shape having an arched portion located across the shoe.
When running a shoe press, the shoe usually is concave and the flexible jacket will be pressed down to the bottom of the shoe when the jacket passes through the roll nip. In a press having a warm or hot counter roll it is important that the jacket does not come into direct contact with the warm/hot roll surface, because the jacket material will not stand the temperature that it will be exposed to if the wet paper web to be pressed will not keep the jacket insulated from the roll surface. Therefore, it has been suggested, see German Utility Model DE-U1 299 02 451 (Valmet) or U.S. Pat. No. 6,158,335, for example, to find out by means of a sensor when a web break occurs in a hot calendering process and to emit a signal from the sensor to a controller, which controls the hot calendering process. Upon a web break, the controller sees to it that the counter roll is lifted away from contact with the shoe press roll, and/or that the press shoe in the shoe press roll is withdrawn sufficiently for the flexible shoe press jacket not to contact the hot counter roll. To make sure that the jacket will not be damaged by heat, it may be necessary to open the press a comparatively long distance if the roll nip is long and the counter roll is of a small diameter. This will require that press actuators as well as a frame mechanism have long strokes. However, it is desirable to use existing press actuators and frame structures.
Earlier it has been proposed, see U.S. Pat. No. 4,563,245 (Voith), for example, to stabilize a rotating flexible roll jacket in a shoe press roll by tensioning it in circumferential direction, so that it is forced inward against a tubular, substantially cylindrical support body, which is supported by and surrounds a support beam for the roll. The cylindrical support body directly supports the press shoe, and the shoe is operated by means of a row of hydraulic actuators, which supported by the support beam act against the inside of the cylindrical support body. The tensioning of the roll jacket is achieved by a tensioning shoe placed diametrically opposed to the press shoe and operated by means of its own hydraulic actuators. Upon forcing the tensioning shoe radially outward against an adjacent portion of the jacket, the other portion of the jacket will be tensioned inward against the cylindrical support body.
German patent DE-A1 32 27 768 (Voith) illustrates that a suction chamber may be used as an alternative to the tensioning shoe. A portion of the jacket is sucked into the suction chamber, so that the jacket will be tensioned against the cylindrical support body.
There also is a suggestion, see German patent DE-A1 199 30 983 (Valmet), for example, to provide expandable support elements inside the shoe press roll. When handling the roll, for example, these support elements are activated to permit lifting of the roll by means of strip-shaped lifting straps, which press against the flexible roll jacket, without the pressure from the weight of the roll causing the lifting straps to do damage to the jacket.
The object of the present invention is to provide a method and an apparatus of the kind initially referred to, which permit the use of existing press actuators and frame structures, at the same time as the opening of the press when necessary still will be ensured, in order to avoid heat damage to the jacket.
In accordance with the invention this object is achieved, in the method initially referred to, by applying a force inside the roll jacket, directed away from the roll nip to tension the roll jacket over the leading edge and the trailing edge of the press shoe, so as to reduce the height of arch of the arched portion over the shoe sufficiently to ensure removal of the jacket from contact with the hot counter roll.
Similarly, this object is achieved, in the apparatus initially referred to, in that second means are provided for applying a force inside the roll jacket, directed away from the roll nip, to tension the roll jacket over the leading edge and the trailing edge of the press shoe, so as to reduce the height of arch of the arched portion over the shoe sufficiently to ensure removal of the jacket from contact with the hot counter roll.
Preferably, the force is applied by means of a tensioning shoe that said second means, which suitably includes a fluid force linear actuator, forces against the inside of the roll jacket. It is also suitable to have said second means carried by a support beam that extends axially through the flexible roll jacket. Thereby, a simple but yet reliable design is achieved.
For cost saving reasons it is suitable that the tensioning shoe consists of bent sheet metal, and it suitably has upturned leading edge and trailing edge.
Preferably, the tensioning shoe has a convex side that upon activation of its associated linear actuator is intended to contact the inside of the flexible jacket. Then, it is suitable that the curvature of the convex side is such thatxe2x80x94when the linear actuator has forced the tensioning shoe outward against the jacket to such an extent that removal of the jacket from the hot counter roll is ensuredxe2x80x94the local pressure between the tensioning shoe and the jacket is of the same order of magnitude over the entire convex side of the tensioning shoe.
The term xe2x80x9cshoe press rollxe2x80x9d is used here to designate a shoe roll having a flexible jacket, irrespective of whether it is used in a press, calender or surface sizing or coating equipment.
In the present context, the expression xe2x80x9cslightly convexxe2x80x9d shall be understood to mean that the radius of curvature of the shoe is larger than that of the flexible jacket, when in an operative shoe press roll the jacket is ready to be pressed against the counter roll by means of the shoe to form the roll nip. Thanks to the differences in radius of curvature, the jacket will get an arched portion straight across the shoe, and the jacket may contact the leading edge and the trailing edge of the shoe. Between these two edges the jacket will have an arch height in relation to the shoe, so that the jacket will contact the counter roll also when the shoe is removed a maximum distance from the counter roll.
The invention will be described more in detail below with reference to the appended drawings.